The present invention pertains in general to a method and an apparatus for testing geophones, and in particular, to a method and an apparatus for testing geophones by means of an analog display of geophone responses.
Seismometers are devices for detecting mechanical vibrations. Seismometers which detect vibration by sensing the movement of an inertial mass relative to a fixed supporting structure are commonly called "geophones". While seismometers may be used in a variety of applications, including earthquake detection, intruder detection, and machinery vibration monitoring, geophones are usually employed in the geophysical exploration for oil. Thus, although the term geophone is used herein with reference to the present invention, the present invention may be usefully applied to other types of seismometers as well.
Within a geophone, an electrically conducting coil is commonly suspended about a magnet and its associated coil pieces such that a spring is attached to each end of the coil. The springs position the coil within the magnetic field of the magnet such that their elastic properties determine the resonant frequency of the system. Application of a vibratory stimulus to a geophone causes the coil to move relative to the magnetic field of the magnet so that a voltage is generated across the coilin response. In order to minimize distortion of a voltage response waveform generated by stimulating a geophone with mechanical vibration, the coil must be centered in order to assure that it cuts the lines of force of magnetic flux of a magnet at an even rate.
Seismic data gathering devices for use with geophones normally have a plurality of geophone channels, each channel having two wires which are connected across the coil of the geophone, and a multi-channel signal recording and computing system connected to each of the channels. Often, up to several hundred geophone channels may be used and each channel may be connected to as many as several hundred geophones. The geophones are manually positioned by moving the geophones and attached connecting wires and by planting each geophone in the earth at a selected location.
It is important to maintain a standard polarity when connecting the geophones together in strings. Once the geophone is planted, the ends of the geophone coil are positioned relative to the surface of the earth such that vibration is initially detected as a negative going output across the channel wires connected to the geophone. Geophones which are connected such that the top of the first geophone is connected to the same channel wire as the bottom of another, produce output waveforms which are 180.degree. out of phase so that the output signals from the geophone cancel each other as a result.
It is also important that the impedence of each geophone channel remain substantially constant over time. Handling of the geophones and their connecting wires may result in damage to the geophones or to the wires. Damaged geophones or geophones with damaged connecting wires may produce a no signal or may produce a distorted signal. When a distorted signal from a damaged geophone is mixed in a recording and computing system with signals from other channels, the mixed signal is distorted and is therefore more difficult to correct or to interpret. Thus, it is desirable to test geophones and geophone strings to ascertain whether they have the appropriate impedence and whether they have been damaged by handling.
Despite the existence of a need to determine whether a geophone is capable of operating properly or not, the testing of geophones has not been standardized. In fact, there are several different types of tests which have been developed for geophones, some of which are peculiar to a particular piece of test equipment and others of which may be implemented by more than one type of equipment.
The resonant frequency of a geophone may be investigated by driving the spring and coil assembly with a constant current oscillator and by observing on the screen of an oscilloscope connected to the geophone the Lissajous pattern indicative of the phase difference between the driving current and the back emf induced in the coil of the geophone as described in U.S. Pat. No. 4,259,563. Impedance may be tested by impressing a test current on each channel in order to develop a test voltage across each channel, by generating high and low reference voltages corresponding to a maximum and a minimum acceptable impedence for each channel, and by comparing the test voltage for each channel with the reference voltages for each channel, as described in U.S. Pat. No. 4,298,969. Coil displacement and resonant frequency may be determined by placing a disassembled geophone in the path of a photocell and by respectively detecting photocell interruption with the coil at rest and after the application of a current pulse as in U.S. Pat. No. 4,448,057.
In U.S. Pat. Nos. 4,257,098 and 4,366,561, three types of tests are performed: a leakage test, for measuring the leakage resistance between a geophone string and ground is performed by applying a voltage to an ouput terminal of a geophone station and by noting any reduction in the supplied voltage due to current leakage from the geophone string to ground; a continuity test, measuring the internal resistance of a geophone string, is performed by passing a current through a geophone string and noting the resulting voltage which is proportional to the internal resistance of the string; and a test indicating response parameters of a geophone string is performed by applying a voltage to the string and by examining the output signal from the geophone as it oscillates in response to the applied voltage. The results of the test in U.S. Pat. Nos. 4,257,098 and 4,366,561 are converted into digital form and transmitted to a computer for analysis and generation of a display notifying an operator that geophone strings are operational or malfunctioning.
As in U.S. Pat. Nos. 4,003,018, 4,015,202, 4,043,175, and 4,052,694, geophones may be stimulated with an impulse current or driven with a periodic sinusoidal current. Resulting voltages, indicative of geophone coil amplitudes, are displayed on a meter with a pointer or on a numerical display, and a GO-NO GO indication is derived on the basis of known specifications of the geophones and cables.
Lastly, a geophone or geophone string may be connected to a seismic recording system, a current pulse supplied, and the responses signal from the geophone or string may be twice differentiated to provide an indication of velocity impulse response or may be utilized in other ways common to deconvolving recorded seismic data as described in U.S. Pat. No. 4,392,213.
Unfortunately, seismic crews have little time to spend in testing geophones. For example, miles of conventional in-line, common depth point (CDP), stackable data are generated per day per crew. Furthermore, a seismic exploration spread may extend for miles and may include thousands of geophones placed over rough terrain so that it is often difficult to provide access for a computer or heavy or bulky equipment to a geophone to be tested. In addition, a large number of geophone types are available so that advance knowledge of the types to be used in a particular occasion, specifications for those types and equipment which may be adjusted to properly evaluate the specifications must be available to an operator in order to properly test the geophones.
In response to practical necessity, existing equipment has been miniaturized to the point that cumbersome but marginally portable devices are available for transportation to sites in order to test geophones. The problem of dealing with a wide variety of geophone types has been approached by providing equipment with modules specific to each type. Of course, the problem still remains that in order to test a geophone, the appropriate module must be available on site.
It is also somewhat of a disadvantage that the existing test equipment provide numerical, pointer or an oscilloscope displays which are only indirectly indicative of the response of a geophone to a stimulus. Insofar as an indirect indicator may not warn of a fault, it is not specifically designed to defect. It would be desirable to have a display which portrays the response of a geophone over every point of a test period in a portable, easily interpretable waveform display.